Some threads depend on information in links and when these links are dead, the threads have no more utility.

Does it make sense to keep them? Or what about orphan threads (threads with 0 replies)?

Cleanup in the forum is a difficult (almost impossible) task but if the users could at least rise a check-flag (mark) when it is obvious that a thread is not helpfull, it would be a bit of a start. At least, this kind of threads would not show up anymore in search results

Re: Dead links and obsolete threads

I understand what you mean about dead links, they are annoying and should be fixed somehow when found if possible. But even a thread or post with a dead link can be useful so we will need to be cautious if we were to delete.

Even threads with zero replies can be useful so I do not think a blanket approach would work.

As for the thread you linked in the above post, I do not see a problem with it. It explains what happened to a valued member and also gives a way to obtain some of the information from his old website.

users could at least rise a check-flag (mark) when it is obvious that a thread is not helpfull, it would be a bit of a start.

Not helpful to who?
Currently there is a way to rate a thread, maybe if users used that option more? I am not sure if that option is tied to the search function. We will have to play with this.